This invention relates to a device for detecting the current flowing in a circuit such as a transmission line. The invention relates more particularly to a device for detecting the off-hook condition of a telephone through the detection of DC current variations occurring on the telephone line.
It is often necessary to detect the off-hook condition of a telephone in order, for example, to actuate a counter used to record the duration of telephone conversations between subscribers, or for various other reasons. More generally, it may be useful to detect certain changes in the value of the current flowing in a telephone line. This can be done by simply inserting an impedance in series with the line and measuring the voltage variations across that impedance. However, this solution raises a number of difficulties in practice. One of these is due to the fact that the added impedance increases the line impedance as seen at the output of the telephone exchange to which the called subscriber is connected, making it necessary, in order to comply with the standards established in relation to the operation of the exchange, to shorten the lines connectable thereto, thereby reducing the extent of the geographical area serviced by that exchange, which is a disadvantage. For example, some telephone lines have an impedance of 120 ohms/km. To comply with existing standards, the total impedance of the line connected to the exchange must be lower than a pre-established value, which limits the length of the line that may be used, hence the radius of the area that may be serviced by the exchange. To insert an additional 100 ohms in series with the line in order to measure the current flowing therein would decrease the length of said radius by close to one km, which is not acceptable.
Also, the circuit using the line current detection information must be dielectrically isolated from that line.
Opto-electronic couplers have been used as a means of resolving this type of problem. In addition to the fact that the light-emitting diode used in such cases causes the line voltage to drop by 1.4 volt, the use of such couplers may entail cost and reliability problems.